1. Field of the Invention
The present invention relates to a voltage measuring apparatus for detecting output voltages of an assembled battery in which a plurality of unit cells are connected in series so as to generate a desired voltage.
2. Background art
For example, a hybrid vehicle has a high voltage battery serving as a driving power source for the motor. In the high voltage battery, for example, a plurality of unit cells for secondary battery (storage battery) such as nickel-hydrogen battery cells and lithium battery cells are connected in series so as to provide a high voltage.
Further, in order that the secondary battery should be prevented from being overcharged or overdischarged, the charging state of each unit cell need to be checked. Thus, in the background art, a plurality (e.g., 55) of unit cells are divided into, for example, five blocks (i.e., each 11 unit cells correspond to one block). Then, the voltage of each block is measured in real time by a voltage measuring IC provided in each block.
In this case, each voltage measuring IC measures the voltages of the unit cells (e.g., 11 unit cells) in the block. Further, an A/D converter owned by the voltage measuring IC converts the detected analog voltage signals into digital signals, and then transmits the signals to a main microcomputer. Then, when voltage abnormality occurs in a block, the main microcomputer generates an alarm so as to notify the voltage abnormality to a crew of the vehicle, or alternatively performs compensation processing for the voltage.
Nevertheless, when fault occurs in at least one of the plurality of voltage measuring ICs, the voltages of the unit cells are not measured accurately. In particular, when fault occurs in the A/D converter provided in each voltage measuring IC, or alternatively when the output voltage of a standard voltage supply used for A/D conversion fluctuates, the measured cell voltages are not converted into accurate digital signals. This causes a problem that the accuracy of measurement of the cell voltages is degraded.
Thus, in the background art, a fault detection device has been proposed for detecting fault occurrence in an A/D converter (see, for example, JP-A-2006-304365). In JP-A-2006-304365, by using an A/D converter, two reference voltages each having a known value are converted into digital signals. Then, when the difference between these voltages increases, it is determined that fault has occurred in the A/D converter.
Nevertheless, in the invention described in JP-A-2006-304365, two reference voltages are converted into digital signals by the same A/D converter. Then, fault occurrence is detected on the basis of the difference between these voltages. This approach causes a disadvantage that in a case that no change arises in the difference value even when fault occurs in the A/D converter, the fault occurrence is not detected.